Up to date, conventional rain sensors have been using the methods of: injecting an optical signal into the windshield to determine the level of rainfall by detecting the decrease in the amount of guided light through the windshield reaching the light receiving element attached to the opposite side of the light source when the raindrops fall on the windshield; transmitting a slanting signal light into the windshield to determine the level of rainfall by detecting the change in the amount of light reaching the light receiving element attached to the opposite side of the light source due to the change in the reflectance when the raindrops fall on the windshield; and attaching serially arranged rain sensors to the windshield to determine the level of rainfall by receiving the light reflected back towards the light source when the raindrops fall on the windshield where the sensors are attached. Among the above described methods, the first method requires a complicated optical system and brings on difficulties in mounting since the optical coupler must be in close contact with the windshield and maintain precise incident angle when the sensing signal light is transmitted into the windshield so as to be guided inside thereof. In the second method, the signal-to-noise ratio (hereinafter referred to as “SNR”) is degraded because the light receiving element receives not only the reflected light from the raindrops bus also the light reflected from the surface of the windshield since the light receiving element is installed in the opposite side of the light source of the sensor. Disadvantage of the third method is a narrow and small sensing area for a light source and light receiving element set since it requires many light source and light receiving element sets to detect raindrops falling on a wide area due to the serial arrangement of the light source and light receiving element.
In summary, a conventional rain sensing method using internal light guiding phenomenon inside of the windshield requires a complicated optical system and brings on difficulties in mounting; in a method measuring reflecting light from the raindrops wherein the light source and the light receiving element are located apart opposing each other, the SNR is degraded due to the reflection from the surface of the windshield; a reflection type method wherein a light source and a light receiving element are serially arranged, has a narrow and small raindrop sensing area.
A pair of wipers is installed on a windshield to overcome a hindrance to visibility caused by rain during rain, and the intermittent speed control of the wipers is accomplished in a step-by-step manner according to the level of rainfall. Since the speed control system for such wipers provides only a couple of steps, it is impossible for a driver to operate the wipers at a desired speed corresponding to the level of rainfall.
Even in the products developed to solve above described problems, the SNR is degraded due to the large amount of reflection from the windshield in addition to the optical signal reflected from the raindrops therefore expected performance in sensing efficiency cannot be achieved.
Furthermore, in a conventional method, optical signals caused by the alternating motion of the wipers are also being received; hence the sensing efficiency is getting worse since it is affected by headlights from another person's vehicle. In other words, during the operation of the wipers headlights from another person's vehicle could be received in addition to the reflected light from the raindrops thereby causing malfunction etc.